1. Technical Field
The present disclosure relates to an illumination light communication apparatus and a communication module which perform a visible light communication by modulating illumination light.
2. Description of the Related Art
Conventionally, a visible light communication in which a signal is transmitted by modulating an intensity of illumination light is proposed for a lighting device which includes a light-emitting diode (LED) as a light source. Such an illumination light communication apparatus transmits a signal by modulating the illumination light as it is, and thus does not require a special device such as an infrared communication apparatus. Moreover, such an illumination light communication apparatus can achieve power saving by using light-emitting diodes as a light source for illumination. Thus, use of the illumination light communication apparatus for ubiquitous information systems in underground malls is contemplated.
FIG. 164A is a diagram illustrating a configuration of an illumination light communication apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2012-69505 (PTL 2). In the circuit, current sensing resistor R3, load circuit 53 which includes three light-emitting diodes, inductor L1, and switching element Q1 are connected in series across DC power supply 51, and turning on/off of switching element Q1 is controlled by control circuit 54. Moreover, smoothing capacitor C3 and rectifier diode D2 are connected across a series circuit formed of load circuit 53 and inductor L1, and form a DC-to-DC converter together with inductor L1 and switching element Q1. A feedback signal is input from constant-current feedback circuit 55 to control circuit 54 to control output current of the DC-to-DC converter so that the output current is brought to be substantially constant. The DC-to-DC converter serves as a constant-current feedback power supply which is controlled so as to supply constant current. Moreover, communication signal S1 is input to control circuit 54. Control circuit 54 modulates load current I1 in a high period of the communication signal by turning on/off switching element Q1 at a high frequency.
FIG. 164B is a diagram illustrating a circuit portion including a specific example of constant-current feedback circuit 55 in FIG. 164A. Constant-current feedback circuit 55 compares, by error amplifier A1, a voltage drop in resistor R3 through which load current I1 flows, with reference source E1 as to whether the voltage drop is higher or lower than reference source E1, and amplifies and outputs a difference between the voltage drop and reference source E1 to control circuit 54. A series circuit which is formed of resistor RA and capacitor C2 and connected between an inverting input terminal and output terminal of error amplifier A1 forms a phase compensation circuit for providing the feedback system with stability. Such a phase compensation circuit, in general, uses a compensation circuit which includes an integrating element to adjust a gain and a phase in an open-loop transfer function. This is known as PI control or PID control of information theory. For example, FIG. 164C is a diagram illustrating an illumination light communication apparatus which includes the average-current sensing circuit disclosed in Japanese Unexamined Patent Application Publication No. 2006-120910 (PTL 1). Integration circuit 56 (formed of resistor R5 and capacitor C3) connected across current sensing resistor R3 can be said to be an output averaging means using the PI control mentioned above.
Moreover, Japanese Unexamined Patent Application Publication No. 2010-283616 (PTL 3) discloses an illumination light communication power supply which provides, through a visible light communication using illumination light, a constant brightness of the illumination light both during a non-communication time where a communications carrier signal is not received and a communication time where a carrier signal is received. The illumination light communication power supply is at a low cost and has high light-source utilization efficiency and high power efficiency.
Further, the illumination light communication apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2011-130557 (PTL 4) includes a DC-to-DC converter having a step-up mode and a step-down mode, a constant-current clamp switch, and a constant-current clamp control circuit. PTL 4 discloses starting limiting a current when a duty cycle of the constant-current clamp switch is greater than a pre-stored maximum (or a minimum), and stopping limiting the current when the duty cycle is less than the maximum (or greater than minimum).
Still further, Japanese Unexamined Patent Application Publication No. 2015-216580 (PTL 5) discloses a dimming-type lighting apparatus which makes flicker inconspicuous even when an amount of light is small, and improves data transmission rate.